Expansion of the invasive European mistletoe in California, USA.
The horticulturist Luther Burbank introduced the European mistletoe (Viscum album L.) to Sebastopol, Sonoma County, California, USA, around 1900 to grow as a Christmas ornament crop and tincture for medicinal use. The mistletoe has since spread from the point of introduction on apple to other hardwood trees, especially non-native hardwoods in yards and farms of the region. Mistletoe surveys were previously conducted in 1971, 1986, and 1991. We re-surveyed the region in 2019, with emphasis on the 1991 perimeter, and documented the current farthest distribution of V. album. This represents a 120-year record of spread. We observed infected trees up to 24.6 km (15.3 miles) from the point of introduction, doubling the farthest distance reported in 1991. The estimated area encompassed by mistletoe-infected hosts increased from 184 km2 (71 miles2) in 1991 to 606 km2 (234 miles2) in 2019. We also updated the host species list including both native and non-native mistletoe hosts. Viscum album spread appears to be limited by available habitat and hosts, but within the survey area it is intensifying, concentrated in urban and semi-urban yards, streets, and farms. However, V. album was also commonly found in riparian areas, which have a large number of native hosts.